<!DOCTYPE HTML>
<html>
<head>
    <script type="text/javascript">
        var num=0,i;
        function timedCount(){
            document.getElementById('txt').value=num;
            num=num+1;
            i=setTimeout(timedCount,1000);
        }
        setTimeout(timedCount,1000);
        function stopCount(){
            clearTimeout(i);
        }
    </script>
</head>
<body>
<form>
    <input type="text" id="txt">
    <input type="button" value="Stop" onClick="stopCount()">
</form>
</body>
</html>